mate-session: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/mate-session - from package: mate-session-manager

You may install the required package with command apt-get install {package}

Error in other languages:
mate-session: bevel nie gevind nie
mate-session: comandă negăsită
mate-session: comando não encontrado
mate-session: commande introuvable
mate-session: command not found
mate-session: käsku ei ole
mate-session: không tìm thấy lệnh
mate-session: komanda nerasta
mate-session: Kommando nicht gefunden.
mate-session: kommandot finns inte
mate-session: komut yok
mate-session: nie znaleziono polecenia
mate-session: níor aimsíodh an t-ordú
mate-session: no se encontró la orden
mate-session: no s'ha trobat l'ordre
mate-session: opdracht niet gevonden
mate-session: parancs nem található
mate-session: perintah tidak ditemukan
mate-session: príkaz nenájdený
mate-session: příkaz nenalezen
mate-session: команда не найдена
mate-session: командата не е открита
mate-session: 命令找不到
mate-session: コマンドが見つかりません